This invention relates to the biodegradation of various nitroaromatic compounds in water and soils, including dinoseb (2-(1-methylpropyl)-4,6-dinitrophenol), using microorganisms.
Certain nitrophenolic compounds are sufficiently toxic to life to render them effective for use as herbicides, insecticides, or miticides. Such compounds include dinoseb (2-(1-methylpropyl)-4,6-dinitrophenol) which has been widely used as a herbicide since the 1950""s on a variety of crops in the United States. Concerns for the safety of agricultural workers has resulted in discontinued use of dinoseb. However, numerous sites remain contaminated with this compound.
Other nitroaromatic compounds are similar to dinoseb in terms of chemical structure, but have other applications, such as in explosives. Such compounds include trinitrotoluene (TNT) and dinitrotoluene (DNT). Because of the widespread use of these compounds over a lengthy period of time, many sites have become contaminated with these compounds, including both manufacturing and military sites.
Many nitroaromatic compounds are either poorly degradable or nondegradable in field environments outside the laboratory. Previously, land farming was the favored method for disposing of these and other chemicals, wherein the chemicals were mixed with soil, fertilizer was added, and the mixture aerated to promote microbial activity. Unfortunately, nitroaromatics were not satisfactorily degraded by land farming or other well-aerated processes. Possible reasons include lack of nitroaromatic-degrading microorganisms, partitioning of the contaminant chemicals to biologically sequestered or inhospitable parts of the environment, and accumulation of toxic partial-breakdown by-products. Problems with land farming in general included the slow rate of biodegradation, high expense, and accumulation of toxic by-products.
Other methods have been used to remove nitroaromatics and similar compounds from contaminated soils, but with little practical success. Such methods include transportation of contaminated soil to hazardous waste dumps, and on-site incineration of the soil. Problems with such methods include high cost and poor accountability of the responsible party.
Previous laboratory studies indicated that certain nitroaromatic molecules are susceptible at least to microbiological transformation. However, the studies did not disclose biochemical mechanisms of such transformation or degradation or whether the nitroaromatic compounds were completely mineralized. In one study, for example, a soil Moraxella microorganism was isolated that was capable of growth on p-nitrophenol as its only source of carbon and energy. Spain et al., Biochem. Biophys. Res. Comm. 88:634-641 (1959). In another study, the anaerobic bacteria Veillonella alkalescens reductively transformed nitroaromatic compounds, converting the nitro groups to amino groups. McCormack et al., Appl. Environ. Microbiol. 31:949-958 (1976).
Aminoaromatic derivatives of nitroaromatics can undergo enzymatic oxidation to form polymeric (large molecular weight) materials. Parris, Residue Revs. 76:1-30 (1980). In the field, such polymers are usually incorporated into soil humic matter. Channon et al., Biochem. J. 38:70-85 (1944); McCormick et al., Appl. Environ. Microbiol. 31:949-958 (1976); Simmons et al., Environ. Sci. Technol. 23:115-121 (1989). Humic matter tends to be long-lived in soils, thereby representing a major long-term environmental fate of many nitroaromatics and aminoaromatics. Other soil microorganisms are capable of cleaving the azo linkages of polymerized aminoaromatics, often forming toxic by-products.
Bacteria are also able to attack nitrobenzoic acid, Cartwright and Cain, Biochem. J. 71:248-261 (1959), as well as o-nitrophenol and m-nitrophenol, Zeyer and Kearney, J. Agric. Food Chem. 32:238-242 (1984), where the nitro group is released as nitrite. Again, however, complete mineralization has not been demonstrated. Further, nitrite release has not been found to be a significant pathway for highly substituted nitroaromatics. No instance is currently known where a compound possessing more than one nitro substituent has been completely mineralized. In fact, the pertinent literature presents no evidence supporting ring cleavage of highly substituted nitroaromatics. Kaplan, xe2x80x9cBiotransformation Pathways of Hazardous Energetic Organo-Nitro Compounds,xe2x80x9d in Biotechnology and Degradation. Adv. Appl. Biotechnol. Ser. 4:155-181, Gulf Pub. Co., Houston, Tex. (1990).
Aromatic groups in general appear to be degradable via only a few aerobic and anaerobic pathways. Gottschalk, Bacterial Metabolism, 2d ed., Springer Verlag, New York (1986), pp. 157-162; Berry et al., Microbiol. Rev. 51:43-59 (1987); Schink, xe2x80x9cPrinciples and Limits of Anaerobic Degradation: Environmental and Technological Aspects,xe2x80x9d in Zinder (ed.), Biology of Anaerobic Microorganisms, Wiley, New York (1988). Aerobically, many aromatic groups are degraded to catechol, protocatechuate or homogentisate by the action of oxygenase and dioxygenase enzymes. Catechol and protocatechuate can be degraded further by aromatic ring cleavage either ortho or meta to the hydroxyl groups. Because of the difficulty of working with anaerobic microorganisms and processes, biochemical pathways describing anaerobic degradation of aromatic compounds have been less well characterized.
Alkyl groups on aromatic rings are degradable via reactions similar to those for simple alkanes. Under aerobic conditions, the terminal carbon is oxidized to yield a carboxylic acid. Degradation then proceeds by xcex2-cleavage to yield either benzoates (odd-numbered carbon chains) or phenylacetates (even-numbered carbon chains). No anaerobic microorganisms capable of carrying out this process have been isolated to date. In spite of the above results known in the art, there is little information currently available on practical means of using microbial cultures to bioremediate nitroaromatic-contaminated soils.
Dinoseb, an intensely yellow-colored compound visible at concentrations as low as 10 ppm, has been found to not significantly accumulate in agricultural soil at normal application rates, even after years of repeated application. Doyle et al., J. Agric. Food Chem. 26:987-989 (1978). However, higher application rates, such as from spills of substantial amounts of the compound, can result in appreciable accumulation at a site. Presumably, therefore, dinoseb at lower concentrations is transformed by certain soil microorganisms. Such transformation appears to result only in the formation of amino and acetoamido forms of dinoseb, which apparently retain significant toxicity. Parris, Residue Revs. 76:1-30 (1980).
Previous work on the biotransformation of the explosive 2,4,6-trinitrotoluene (TNT) indicates that the primary mode involves transformation (reduction) of the nitro groups. Kaplan, supra. A recent paper from Soviet researchers describes degradation of TNT by a strain of Pseudomonas fluorescens. Naumova et al., Mikrobiologiya 57:218 (1988). But, while these reports shed some light on microbial events and hypothetical biochemical mechanisms therefor, they neither disclose nor suggest effective methods for bioremediating soils or wastewater contaminated with these compounds. Further, the Soviet results have not been confirmed outside the U.S.S.R.
Hence, although several anaerobic microbiological systems have been described for degrading other aromatic chemicals, little to no information is available on practical means of using these cultures to bioremediate contaminated soils and waters, especially soils and waters contaminated with nitroaromatics. In today""s world, effective remediation of environmental sites contaminated with compounds, such as nitroaromatics, requires that the contaminants be completely mineralized to ensure the absence of latently toxic by-products. Such results for nitroaromatics simply have not been shown in the prior art, particularly as applicable to large-scale, low-cost bioremediation efforts.
Therefore, there remains a need for a method to effectively bioremediate dinoseb-contaminated soils, as well as soils contaminated with other nitroaromatic compounds, such as TNT and DNT.
Further, there is a need for such a method that can be performed at a natural site contaminated with dinoseb or a related nitroaromatic compound.
Further, there is a need for such a method that can completely degrade dinoseb and other nitroaromatics, leaving no detectable or environmentally significant amounts of aromatic by-products or other toxic intermediary compounds, including polymeric derivatives.
Further, there is a need for such a method employing microorganisms of types and species profiles normally found in many soil environments.
Further, there is a need for such a method that is inexpensive and easy to perform, particularly on a large-scale, in the field.
Further, there is a need for such a method that can be performed rapidly, including in the field.
Further, there is a need for such a method that will effect bioremediation of nitroaromatic-contaminated soil without specialized bioreactors or other complex equipment.
In accordance with the present invention, soil or water contaminated with one or more nitroaromatic compounds is subjected to a two-stage bioremediation process employing different microorganisms during each stage. The stages comprise an initial fermentation stage followed by an anaerobic stage. Most of the actual biodegradation of the contaminant nitroaromatics takes place in the anaerobic stage. At the end of the anaerobic stage, the contaminant nitroaromatics have been biodegraded to non-toxic end products.
As another aspect of the invention, complete biodegradation of nitroaromatics has been found to occur only under anaerobic conditions. Since anaerobiosis generally requires an aqueous environment, it is usually necessary to add extraneous water to a nitroaromatic-contaminated soil to create a fluid mud slurry of the soil before beginning the process. Contaminated water can be subjected to the process directly.
During the first stage of the process, the normally aerobic contaminated soil (with water added to form a mud slurry) or contaminated water alone is rendered anaerobic. The preferred method for achieving an anaerobic condition is by a fermentation of a supply of starchy carbohydrate or other readily fermentable carbon source added to the slurry or water. Fermentation, where the carbon source is a starchy carbohydrate, is performed by one or more species of aerobic or facultatively anaerobic amylolytic microorganisms inoculated into the slurry or water. Amylolytic microorganisms are not required if the carbon source is a simple sugar, such as fructose or glucose.
As another aspect of the invention, the aerobic or facultative microorganisms are preferably isolated and enriched in a culture containing the particular nitroaromatic present in the contaminated soil or water.
As another aspect of the invention, it is usually necessary at the beginning of the aerobic stage to add an extraneous source of nitrogen for the microorganisms. The nitrogen source is preferably in the form of ammonium ion or simple amino compounds readily utilizable by aerobic and anaerobic microorganisms.
As another aspect of the invention, it is preferable to stimulate a rapid onset of intense fermentation to quickly cause exhaustion of the oxygen dissolved in the slurry or water, thereby rendering the slurry or water anaerobic, without exhausting the carbon source. Quick attainment of anaerobiosis minimizes oxidative polymerization of any amino derivatives of the nitroaromatics that tend to form under aerobic conditions. Once formed, such polymers are difficult to biodegrade. Rapid anaerobiosis can be achieved by inoculating the slurry or water with a large dosage of aerobic and/or facultative fermentative microorganisms.
If necessary, mineral nutrients, including phosphate salts, can be added to the soil slurry or water to facilitate microbial growth. Supplementary vitamins and cofactors may also be required, but probably only when treating wastewaters having very little dissolved organic carbon. Soils generally have sufficient nutrients.
As another aspect of the invention, the carbon source added to the soil slurry or water for aerobic fermentation is preferably a starchy carbohydrate substance hydrolyzable to constituent sugars by amylolytic microorganisms in the aerobic inoculum. A starch is preferred over merely adding free sugar because the starch serves as a reservoir of metabolizable carbohydrate that ensures an adequate supply of easily metabolizable sugar for the microorganisms, both in the aerobic stage and in the anaerobic stage. Anaerobic biodegradation of nitroaromatics requires such a sustained sugar supply. A supply of sugar added at the beginning of the aerobic stage would generally be exhausted prematurely, making it difficult to maintain anaerobic conditions for the requisite amount of time to achieve complete mineralization of the nitroaromatics.
As another aspect of the invention, the amount of starchy carbohydrate to be added to the volume of soil slurry or water to be treated can be xe2x80x9ctailoredxe2x80x9d to ensure the desired degree of biodegradation is attained and waste of carbohydrate is avoided. The amount of carbohydrate should be just sufficient to supply the metabolic needs of the microorganisms until the anaerobic biodegradation of contaminant nitroaromatics is complete.
As yet another aspect of the invention, once strict anaerobic conditions have been attained in the volume of slurry or water, an inoculum comprised of an anaerobic consortium of microorganisms is added to the volume to start the second, or anaerobic, stage. Anaerobic conditions are preferably determined via a potentiometric measurement, where a redox potential of xe2x88x92200 mV or less indicates strict anaerobic conditions.
As yet another aspect of the invention, the anaerobic consortium comprises multiple species of microorganisms that have been grown in a medium containing one or more nitroaromatics identical or similar to the contaminant nitroaromatics to be biodegraded. The anaerobic microorganisms are able to biodegrade the nitroaromatics in the presence of metabolizable sugar to simple non-toxic compounds, such as methane, carbon dioxide, and acetate.
As yet another aspect of the invention, after inoculation, the anaerobic consortium is afforded sufficient time to biodegrade the contaminant nitroaromatics in the soil slurry or water to non-toxic end-products. Because degradation of the nitroaromatic compounds occurs in an anaerobic environment, polymerization to toxic humic-like compounds and other large, long-lived, latently toxic molecules normally formed in aerobic environments is prevented.
As yet another aspect of the invention, the present method is preferably performed in a suitably large covered vessel for containing the contaminated soil slurry or water during bioremediation. Such containment ensures that anaerobic conditions in the soil slurry or water are reached more rapidly and are better controlled and maintained. Containment also facilitates easier control of environmental parameters, such as temperature, pH, and, if needed, escape of volatile gases from the slurry or water being treated.
It is accordingly one object of the present invention to provide a method for effectively bioremediating soils and waters contaminated with one or more nitroaromatic compounds.
Another object of the present invention is to provide such a method that can be performed at natural sites contaminated with nitroaromatics.
Another object of the present invention is to provide such a method that will allow contaminant nitroaromatics in soil or water to be biodegraded to such an extent that no detectable or environmentally significant amounts of aromatic by-products or other toxic intermediary compounds are left in the soil or water, including latently toxic polymeric derivatives of the nitroaromatics.
Another object of the present invention is to provide such a method that utilizes microorganisms similar to those found in many soil and aquatic environments.
Another object is to provide such a method that is inexpensive and easy to perform, particularly on a large scale in the field.
Another object is to provide such a method that can be performed rapidly, even in the field.
These and other objects, features, and advantages of the present invention will become apparent with reference to the following description and drawing.